Vitamins are complex organic (carbon-containing) substances, with zero caloric value and no appreciable weight in the body. Nevertheless, the body cannot function without them: they are necessary to support growth and maintain health. The main mechanism of the biological function of vitamins is as a part of body enzymes. Vitamins act as coenzymes which, attached to a protein molecule, form enzymes. They are necessary for the metabolism of nutrients, cellular function, bone and tissue formation and regeneration, glandular, neural and hormonal function - in short, for life.

With a few partial exceptions, the body cannot synthesize vitamins on its own, and depends on dietary intake or supplementation. There are 13 vitamins among essential nutrients; A, B1, B2, B3, B5, B6, B7, B9, B12, C, D, E and K. Four of them are fat-soluble (A, D, E and K), and rest of them water-soluble (C and B-complex). There are also at least several other vitamins that are not considered essential, and are classified as accessory nutrients (inositol, choline, PABA, pangamic acid and bioflavonoids).

Excess amounts of most water-soluble vitamins (B12 being one exception) are being excreted daily, thus they require regular daily intake for optimum health. They are also generally more fragile, easily destroyed by heat and air (oxidation), and lost to a greater degree during food processing and preparation.

Fat-soluble vitamins require presence of dietary fats, or bile salts, for absorption. They are, in general, stored by the body and don't have to be supplied as continuously as water-soluble vitamins. At the same time, they accumulate in the body more easily, which makes them generally more toxic when taken in excess.

Vitamins, as all other nutrients, need to be present in the body at a near-optimum level. The excess - especially in the form of individual vitamin excesses - can be as detrimental to health as deficiency. Thus the optimum diet needs to provide sufficient and balanced intake of all vitamins, preferably from food. This, however, may not be sufficient; supplementation is usually necessary to achieve their optimum intake.

Vitamin supplementation

Food and Nutrition Board's Dietary Reference Intakes (DRI), which replaced the old RDA (Recommended Daily Allowances), gives what amounts to a reference point for health-supporting intake of nutrients, including vitamins. These intake levels are given for all age groups, the two genders, as well as for pregnancy and lactation.

One needs to keep in mind, though, that those values are based on the average levels of nutrients found in individuals that appear healthy. Thus the DRI values should be looked at as the average intake levels of nutrients needed to prevent short-term signs of deficiency - not necessarily their optimum long-term levels.

Your current individual need for any nutrient - including vitamins - can vary significantly from the DRI value, depending on your health condition, lifestyle, nutritional imbalance, intake of medications, herbal and other preparations, and so on. In general, your optimum vitamin intake is likely to be higher than DRI values, not seldom significantly.

While a broad, balanced nutritional supplementation that includes vitamins is beneficial, it is unlikely to correct serious nutritional imbalances, or ailments and diseases resulting from them. It requires supplementation tailored to your specific nutritional profile, obtained from appropriate lab tests. R

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